That's the message of an eye-popping report from researchers at the US Geological Survey. This chart tells the story:To understand it, note that in the arid Central Valley, farmers get water to irrigate their crops in two ways. The first is through massive, government-built projects that deliver melted snow from the Sierra Nevada mountains. The second is by digging wells into the ground and pumping water from the region's ancient aquifers. In theory, the aquifer water serves as a buffer—it keeps farming humming when (as has happened the last three years) the winter snows don't come. When the snows return, the theory goes, irrigation water flows anew through canals, and the aquifers are allowed to refill.